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any member of more than 65,000 animal species belonging to the class Gastropoda, the largest group in the phylum Mollusca. The class is made up of the snails, which have a shell into which the animal can generally withdraw, and the slugs —snails whose shells have been reduced to an internal...

Large and small animals now affect humans by competing with them or with their livestock. They may prey on people or on livestock or carry diseases affecting either. The bilharzia snail and the Simulium fly (host to an organism causing blindness), the tsetse fly, and the mosquito collectively affect human beings and their livestock far more than do such individually large or formidable...

The nervous systems of the more primitive mollusks (snails, slugs, and bivalves, such as clams and mussels) conform to the basic annelid plan but are modified to conform with the unusual anatomy of these animals. In snails a pair of cerebral ganglia constitutes the brain, which overlies the esophagus. Nerves leave the brain anteriorly to supply the eyes, tentacles, and a pair of buccal ganglia....

A classic example of habituation is the following observation on the snail Helix albolabris. If the snail is moving along a wooden surface, it will immediately withdraw into its shell if the experimenter taps on the surface. It emerges after a pause, only to withdraw again if the tap is repeated. But continued repetition of the same tapping at regular intervals elicits a briefer and more...

...this type and hence may overlook differently coloured prey of the same species. The phenomenon—known as a perceptual set or a search image—is exemplified by the predator of the European snail Cepaea. Predators encounter one morph and form a search image; they continue to hunt for that one form until its increasing rarity causes the predator to hunt randomly, encounter a...

The snails and slugs include hermaphroditic as well as dioecious species. Copulation in the hermaphroditic land snail Helix is preceded by a curious courtship involving a bizarre tactile stimulation. When the two partners come together, each drives a calcareous dart (the so-called love dart) into the body wall of the other with such force that it is buried deep in the other’s internal...

...of mollusks are aquatic and excrete nitrogen in the form of ammonia. In octopuses, however, nitrogen is excreted as ammonium chloride, which is quite strongly concentrated in the urine. Terrestrial snails and slugs excrete uric acid but may also excrete ammonia when living in moist surroundings.

...which causes acid products to accumulate; when normal conditions are restored, the animals increase their ventilation and oxygen extraction in order to rid themselves of the acid products. In snails, the feeding mechanism is independent of the respiratory surface. A portion of the mantle cavity in the form of a gill or “lung” serves as a gas-exchange site. In air-breathing...

...macrostomum, resides principally in the intestine of songbirds. The eggs of the parasite pass to the outside in the feces of the birds and are readily ingested by a terrestrial snail, Succinea, an inhabitant of waterlogged meadows and riverbanks. The parasite eggs hatch into the first larval form within the snail. The next stage, called the sporocyst, is strikingly...

The sessile marine snail Vermetus gigas secretes mucus strings up to 30 centimetres (12 inches) long that extend away from the shell and entangle fine plankton. At intervals, the strings are drawn back toward the mouth and swallowed.

...responses are reflexes evoked by very specific stimuli; rapid swimming by cockles clapping their shells, for example, is elicited by starfish extract. Shell jerking is produced in a freshwater snail (Physa) by contact with a leech, another specific response to a major predator.

...protein in the gastrointestinal tract. Zinc deficiency in nut-bearing and fruit trees causes such diseases as pecan rosette, little leaf, and mottle leaf. Zinc functions in the hemosycotypsin of snails’ blood to transport oxygen in a way analogous to iron in the hemoglobin of human blood.

...in Africa, southern Europe, and the Middle East. Eggs, laid in the veins, break through the vein wall into the bladder and are voided during urination. The larval fluke develops in the body of a snail (chiefly of the genera Bulinus and Physopsis), the intermediate host. The mature larva makes its way into the body of the final host, man, through the skin or the mouth.

...is poor due to poverty or due to the lack of infrastructure to support adequate health care services. The disease is ordinarily contracted by working, bathing, or swimming in water populated by snails that carry the worms. The parasites were first identified as a cause of the disease in the 1850s by Theodor Bilharz, a German pathologist working in Egypt.

...and sheep. The eggs pass through the bile duct and are excreted in the feces. If the eggs get into pools of water, they hatch after a few weeks, and the larvae must find their way into a small water snail. There, in the course of about two months, they multiply and emerge as free-swimming larvae. These finally attach themselves as cysts to grass or leaves of plants growing in the water. The...

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Snails and slugs are similar animals. The main difference between them is that a snail has a shell and a slug does not. Snails and slugs belong to the group of soft-bodied animals called mollusks, which also includes oysters, clams, and squid.

There are more than 40,000 different species of snails and slugs throughout the world, and they are remarkably well adapted to survival. They may be found almost everywhere-on land, in trees, in freshwater ponds and streams, and in salt water from shoreline to ocean depths. Snails have shells; slugs do not.